Furnaces have been available as two-stage or modulating gas-fired types using a modulating or two-stage gas valve and a tapped winding circulating air blower motor. At reduced gas input levels, however, these furnaces operate at excess combustion air and hence at a reduced heating efficiency. Additionally, to maintain a reasonable furnace temperature rise, the blower speed is also reduced. Where a tapped winding, shaded pole or PSC blower motor is used, the efficiency decreases at reduced speeds and the result of a reduced output is a reduced efficiency.
An electrically commutative blower motor (ECM) has an increased electrical efficiency at reduced speeds such that a furnace can provide reduced outputs with no sacrifice in electrical efficiency. The electrical efficiency is, however, only one parameter. The air-fuel ratio is a constant for all burning rates but the combustion air must be supplied in a constant ratio to the fuel by regulating the inducer fan or the manifold gas pressure. When an ECM is used to drive the blower, the increased electrical efficiency with lowering speed must be balanced against the temperature rise in the heat exchanger. The temperature rise in the heat exchanger is normally limited to a maximum 200.degree. F. discharged but an increased discharge temperature has a penalty of decreased heat exchanger life. A reduced temperature rise can result in condensation of the combustion products in the heat exchanger which is permitted by the American Gas Association only upon startup. Also, the temperature rise affects the amount of air circulated and the efficiency of the combustion process.
In addition to balancing out electrical efficiency, the temperature rise, air flow, cycle time and temperature of the discharge air, certain features are desired. Low heat is quiet, provides heat during low demand, and lengthens thermostat cycle time thus decreasing thermal droop. High heat provides heat during high demand and provides preheating of heat exchangers for low fire operation. This all translates into a better comfort level by operating the furnace according to the current needs.